Additives play a crucial role in the performance of polymeric materials, particularly polymeric foams, and are even more important in determining their properties. However, certain desirable additives may cause difficulties in the processing, the use and/or the disposal of polymeric materials as a result of the reactivity and cross-reactivity of the additives.
For instance, infrared attenuation agents are very effective in increasing the extinction coefficient, thus increasing the R-value of polymeric foams. However, many infrared attenuation agents are both inorganic and hydrophilic, which makes it difficult to disperse them in polymeric compositions. Other infrared attenuation agents may be very reactive with other additives often used in plastics, such as iron oxide and hexabromocyclododecane (HBCD), a flame retardant. Another important property for polymeric compositions is ultraviolet light stability. However, HBCD, for instance, increases the sensitivity of polystyrene foams to ultraviolet light.
Brominated flame retardants, such as HBCD, have been used extensively in extruded polystyrene (XPS) foams. However, brominated flame retardants are thought to cause bioaccumulation and ecotoxicity problems. Some Europeans countries, such as Sweden, totally ban the use of HBCD due to the potential for bioaccumulation and toxicity to aquatic organisms.
Additives may also impact the processing of polymeric materials. For instance, HBCD acts as a plasticizer, which tremendously decreases the strength of XPS foam products that incorporate it. In order to compensate for the weakening effects of HBCD or other additives that exhibit a plasticizer activity, additional material will be required in the form of thicker cell walls and struts to maintain the target strength of such foams, increasing both the density and the cost of the resulting products. Further, HBCD can decompose at higher processing temperatures, adversely affecting not only the product but also processing machinery, such as extrusion dies, barrels and screws.
Microencapsulation is a well developed technology that has been employed in many different fields. U.S. Pat. No. 3,660,321, for example, discloses shaped solid polystyrene articles comprising microcapsules containing flame retardant and having diameters of 20 microns (Example 1).
U.S. Pat. No. 4,138,356 teaches that microcapsules having an average diameter below 5 microns and containing flame retardant can be incorporated into polymeric materials such as polyurethane foam without affecting the structural integrity of the cell walls of the foam.
Example A of U.S. Pat. No. 5,043,218 discloses coating HBCD with a melamine:formaldehyde polymer to form microencapsulated HBCD having a mean particle size of 7.5 microns. This patent also teaches that polystyrene foams containing such microcapsules can be made using hydrocarbon blowing agents. European Patent No. 180795 discloses flame retardant agents comprising ammonium polyphosphate microencapsulated within a melamine formaldehyde resin.